The present invention relates to automatic electric circuit breakers, and particularly to a circuit breaker having increased current interrupting capacities.
Manufacturers of automatic electric circuit breakers are increasingly being faced with the task of developing new circuit breaker designs and upgrading existing circuit breaker designs to achieve higher current interrupting capacities in order to cope with the ever increasing current availables posed by modern utilities. That is, due to the updating of electrical utility equipment, the magnitude of current available to feed a circuit fault has increased significantly, and it becomes the task of the circuit breaker protecting the faulted circuit to safely interrupt this higher fault current. Thus, the interrupting capacity of a circuit breaker has become a most critical performance parameter.
The most direct approach to achieving increased interrupting capacity is to provide a fast acting circuit breaker. That is, a circuit breaker will have an easier interrupting task if it is capable of effecting and maintaining contact separation early in a fault current wavefront and thereby begin developing an arc voltage in opposition to the fault current driving voltage before the fault current achieves its prospective peak. This is due to the fact that such early development of an arc voltage is effective in cresting the fault current at an actual peak amplitude which is less than its prospective peak amplitude.
When a typical circuit breaker is subjected to high level fault currents, the movable contact arms can be literally blown away from their closed positions by the electrodynamic forces associated with such high level fault currents. This contact separation or so-called "pop" can occur relatively early in the fault current wavefront. Unfortunately, unless the breaker trip unit can act to trip the breaker before the first current zero, i.e., within the first half cycle of the fault current wave, the contact gap is reduced and, in fact, the contacts may even re-engage. The arcs drawn between the contacts as they pop do not contribute to the ultimate interruption which can only begin as the circuit breaker is tripped. Rather, these spurious arcs have deleterious effects on the circuit breaker. They needlessly erode contact material. The arc chutes are contaminated with ionized gases which degrade their dielectric strength and thus jeopardize their ability to extinguish the arcs drawn between the contacts when the circuit breaker is subsequently tripped to commence interruption of the circuit. Moreover, these spurious arcs contribute to an excessive build-up of internal gas pressures often sufficient to rupture the circuit breaker molded case.
It would be highly advantageous to utilize this "popping" motion of the breaker movable contact arms to trigger the final interruption of high level fault currents. That is, if the contact gap created by electrodynamic forces associated with the fault current could be not only sustained but rapidly increased to that gap produced when the arms achieve their final circuit interrupting position, the prospects for a successful interrupt are materially enhanced. In this case, the arcs associated with contact pop are not spurious, but in fact begin the requisite rapid build-up of arc voltage in opposition to the system driving voltage in current limiting fashion. U.S. Pat. Nos. 3,505,622 and 3,534,305 contain disclosures of prior approaches to this end.
It is accordingly an object of the present invention to provide an automatic electric circuit breaker having materially improved current interrupting capacity.
A further object is to provide a circuit breaker of the above character wherein the popping of the breaker contacts is utilized as a beneficial, contributing factor in the successful interruption of a high level fault current.
An additional object is to provide a circuit breaker of the above character wherein popping of the breaker contacts triggers a circuit breaker trip early in the wavefront of a high level fault current.
Another object is to provide a circuit breaker of the above character wherein the facility to trip the circuit breaker in response to popping of the breaker contacts can be economically and efficiently incorporated in circuit breakers of existing design.
Other objects of the invention will in part be obvious and in part appear hereinafter.